They are roll pins and the usual practice is not to re-use roll pins since the "spring has sprung" and they have lost the tension that holds them in. They are usually so cheap that gunsmiths who work regularly with guns having roll pins will have a bunch of them, and they can be ordered from Brownells or, for a specific gun, from parts places like Gun Parts Corp.

Also, always use a proper roll pin punch; a standard flat end punch can mess up the roll pin and wedge it in place, damaging the gun.